Durable water repellent finishes are hydrophobic coatings that are applied to textiles to make them water-resistant. However, despite the name, most durable water repellent finishes on the market tend to wear off with time and show very poor durability in harsh conditions (e.g. a textile washing process or in an acidic environment). Several inventions have been developed in order to obtain textiles with water repellent properties. The known methods give however textiles with poor wear resistance and washing durability and are somewhat difficult to apply onto the material. The reason for the unacceptable wash resistance of the mentioned systems is the weak attachment of the hydrophobizing agent to the textile fibers. Hence, to achieve persistent water-repellency in textiles, it is imperative for the water-repellent coating to form strong covalent bonds to the fibers, with the additional requirement of them withstanding the harsh conditions in e.g. house-hold washing machines or out-doors. While obvious, it is not a straight forward matter to produce these bonds, as very few, if any, hydrophobizing compositions (for treatment of e.g. cellulosic fabrics) fulfill the combined requirements of formation of covalent bonds that withstand high alkaline conditions (pH>10) at high temperatures (≥40 C.°), industrial relevant application times of the formulation, acceptable cost and a non-hazardous application process; not to mention non or little impact on the environment.
For decades, the application of fluorine compounds has been the route of choice in the manufacturing of e.g. water-repellent clothing. The success of fluorocarbons is due to their extreme hydrophobicity and oleophobicity, their extremely low surface tension and their tendency to remain on the textile. But despite its proven effectiveness, the application of “fluorocarbons” is highly controversial, due to its negative impact on the environment and to studies1 on animals showing that accumulated fluorocarbons (in the body) can be extremely hazardous.
The potential of (non-fluorinated) organosilanes, which are derivatives of silanes containing at least one carbon to silicon bond, to render water repellency to surfaces has been known for some time. Particularly, the so called alkoxysilanes and silicon halides (which have at least one hydrophobic moiety and one to three hydrolysable alkoxy and halide groups respectively) have long been regarded as potential candidates for rendering water-repellency to hydroxyl bearing surfaces (e.g. cellulose containing materials). This view is based on the knowledge that in the presence of water the reactive groups (alkoxy/halides) of organosilanes hydrolyze to form hydroxyl (OH) groups. These groups (in theory) promote the adsorption of the silanes to the OH-bearing surface through a hydrogen bonding mechanism. The silanes can subsequently be covalently attached to the surface through a heating process leading to the release of water.
However, the hydrolyzed monomeric organosilanes are highly reactive and are known to undergo condensation reactions (in the solution), leading to the formation of polymeric structures (and gels during prolonged times). Also, the high reactivity of the mentioned silanes (specially the silicon halides) requires the undertaking of highly complex measures (to ensure “dry” water-free conditions) to maintain the silanes in their reactive form, something that up to date has been considered as too complicated and not economical. Accordingly, there are very few patents available that deal with the hydrophobization of textile with (non-fluorinated) organosilanes. Even fewer patents put emphasis on the resulting washability/durability. From the above it is clear that non-fluorinated durable water repellent finishes have to be/remain firmly attached to the substrate, especially under “normal” washing conditions, before they can be regarded as possible alternatives for the “fluorine equivalents”. It is further clear that employing organosilanes is not an easy task due to their high reactivity, which to the best knowledge of the inventors up to date has made their application impractical. What is also unattractive from an economical, environmental and practical point of view is the use of organic solvents, such as alcohols, in the application of organosilanes or fluorocarbons. Further, the application of organosilanes must be affordable, industrially feasible, and be more environmentally friendly than the existing “fluorine coating” processes. Additionally, hazardous solvents should be avoided and instead the use of water promoted. The general objective in preparing a water based composition for rendering textile water repellent, is to devise a system in which the reactivity of the organosilane is maintained over a long period of time, making use and storage of said composition practically, economically and environmentally feasible.
It is also important that the process is more environmentally friendly than the current silane processes, which comprises use of different solvents. There is also a need for a durable coating which stays on the fabric after washing. Currently there is no hydrophobization process which fulfills all (or at least the majority) of the stated requirements. This has been the motivation for the work leading to this invention. In what follows, a novel organosilane composition together with its application process, which is believed to fulfill the stated requirements, will be presented.